Free-piston air-compressors



ct. 30, 1956 E, S, L, BEALE FREE-PISTON AIR-COMPRESSORS 3 Sheets-Sheet 1 Filed Aug. v30, 1955 ATTORNEY Oct. 30, 1956l E. s. l.. BEALE FREE-PISTON AIR-COMPRESSORS Filed Aug. 30, 1955 ATTORNE Y Oct. `30, 1956 E. s. L. BEALE 2,768,781

FREE-PISTON AIR-COMPRESSORS Filed Aug. 30,- 1955 3 Sheets-Sheet 3 Y t By I ATTORNEY United States;

FREE-PISTON AIR-'COMPRESSORS Application August` 30, 1955, Serial No. 531,537

Claims priority, applicationV Great Britain September 9,'195k4 4` claims. (ci. 23a- 23) The present invention relates. to free-piston air-compressors, that is to say air-compressors in which an engine pistonworking in an engine cylinder has coupled thereto a compressor piston working in a compressor cylinder, energy stored in the compressor cylinder by the compression of air during each working stroke of the engine piston serving to drive the engine piston on its compression stroke, and the compressor cylinder being in communication with-a compressed fair reservoir through one or more delivery valves;

When such a compressor is started'with the air in the reservoir at atmospheric pressure, it is necessary to block the delivery valve or valves in order to maintain suicient pressure in the compressor cylinder to return the engine piston on its compression stroke. Various means have been usedzor` proposed for this purpose, certain of which are, for example, described in British patent specilications Nos.` 373,488 and 391,676. These and most other` known blocking means are designed to operate with plate-type delivery valves in which the amount of lift of the plate is relatively` small, for example about 2 mm., land the required movement of the blocking.` meansbetween its operative and Ainoperative condition correspondingly small.

Another ytype of delivery valve, sometimesknown as astreamlinetype valve, has come into use and has advantages over the older plateatype valve. In the streamline-typel valve `a i number of thin= steel lamin'fae are clamped along ,nel edge andrthe laminaefare arranged-:to tlexbetween a 4closedposition` in which they bearupon`l suitablerseatings and an open position iny which;they.areV

supported-by;stoppla-tes.v The planes off the laminae are approximately.parallelgto` the direction of'air How.v EX- amples of; such-:valves are disclosed in British patent.

speciiicationsvNos. 5'80gl7l anda706,354.

With streamline valves the area to be blocked is the whole-,outletyareafatthefacepofthe'valve andin order to avoid unduly restrictingtheV outlet when unblocked a much-larger movement yofthe blocking meansbetween the blocked andfrfully unblockedV positions isiv required; for instance,about;r lirnm;` lnfaddition, Athe forcewhichthas the Aothenhand it must :be .arranged .that blocking 'cannotf occur under., any. normal running conditionsof the com- PlSSOR The .-prlesentinyentionhas for its object to--provide-improvedf blocking means, suitable forA use VWithrstreamlinetypegvalves, in'gwhichthe4 force exertedv duringvzfblocking:

and the extent of movement can be relatively large, in which;the'transitionf-pfromr.theblockedkto the fully unblockedg conditionftakes. place .vwithv' a relativelyn small` change; .in.press ur. inthe=reservoir, andrin whichthe change from the fully un-blocked to the blocked condi- Alatent ICC 2. tion can be arranged to take place at a substantially lower reservoir pressure than the change from the blocked tothe fully unblocked condition.

According to the present invention, there is provided a free-piston air`-compressor having one or more delivery valves controllingV the delivery of air from a compressor cylinder to a reservoir, means for blocking the one or more delivery valves by the application of a suitable torce to the blocking-means, land automatically-operating blockingl control means for un-blocking` the valves by removal ofthe said force, the blocking control means comprising two pistons of different diameters each working in a cylinder, the pistons being coupled together and in their blocking positionv applying, or permitting4 the application of, the said force to the blocking means and in theirun-blocking position removing, or permitting the removalof, the said force, spring means urging the pistons toward their blocking position, the front face of the-larger-piston being subjected to the pressure of the reservoir, pressure on this face tending to move the pistons toward their un-blockingposition, the rear face of one of thepistons being in communication with* the atmosphere, and means whereby when the pistons arein their blockingposition the rear face of the other of the pistonsV is subjected to the pressure of'thereservoirand whereby in response to a predetermined increaseV in the reservoir pressure the pistons Vmove'into their unblocking position inw'hich the rear-face of theV said other piston is subjected to atmospheric pressure.

Preferably the rear face of'the larger piston is in communication with the atmosphere, the rear face of the smaller piston being subjected tothe reservoirl pressure when in the blocking position and to atmospheric pressure when in the un-blocking position.

The invention will be describedy by way of example-with reference to the accompanying drawings in'rwhich Y Fig. l isfa view in sectionalielevation of part of a freepiston aircompressor embodying-the invention,

Fig.- 2is a view in section on the line 2 2' of Fig; 1 and Fig. 3 is a diagrammatic representation of'another embodimentof the invention.

Referring to Figs. l and 2, acompressor cylinder 10 has an end coverfplate 11 secured by studs 12, the cover plate ll'h'aving water-coolingpassages-laand two delivery valves.14and`15 which are of the streamline type already referred to. The valves Vare secured'in apertures inA the -cover platell by means ofya clamping plate 16 and studs 17./ These valves-control the delivery of air from the cylinderltlto reservoirs which are notf shown but are-connected to the delivery passages'lS'fformed in the compressor casing-19.

For the Vpurpose of blocking the delivery valves 14 and 15 there are provided two blocking plates24 and 25 which are slidable perpendicularto the paper in Fig. 2 and are guidedb'y elongated studs Zo'passing through apertureslinfthe plates 24 and '25;VV A springtnot shown) surroundseach stud 26and is locatedbetween the blockingplateand the clamping plate 16'thus tending tourg'e the blocking plates away from the face of the valve andV thus into vtheun-'blocked position.

The two blocking plates 24and25r'are yoked together by a bridgepiece 27. The blocking .plates are rockably mounted upon the bridge piece 27 and a short springv (not shown) is provided under each nut 284 which screwsonto a stud threaded into thev blocking. plate. The rocking is permitted by providing adequaterclearance between thisfstud :and the aperture in theA blockingpl'a'te throughwhich it passes; This Vrockable -mounting en- -ables the blocking plates to seat snugly:` against thet faces of the valves.-

The means for exerting force upon the blocking plates 24 and 25 in order to block the valves 14 and 15 are as follows. The end of the casing 19 shown in Fig. 1 is closed by a cover 20 which comprises an inwardlyprojecting hollow boss 21 the innermost end 22 of which constitutes the cylinder for the larger piston 23 hereinbefore referred to. The smaller piston 29 is fixed to the larger piston 23 by a bolt 30 threaded into a piston rod 31 which is slidable within an aperture 32 formed in an inwardly projecting boss 33 forming part of an annular member 34 lixed to the cover 20 by studs 35. The inner end prart 36 of the boss 33 constitutes the cylinder for the smaller piston 29. The bolt 30 is centrally apertured, this aperture connecting with an aperture 37 in the bolt head.

A strong spring 38 is provided to urge the piston assembly 23, 29, 31 to the left in Fig. l, thus exerting pressure through the head of the bolt 30 upon the bridge piece 27 and thus upon the two blocking plates 24 and 25.

The front face of the larger piston 23, that is the left face in Fig. l, is always subjected to the pressure in the reservoir which is connected to the space within the cover 20. The rear face of this piston and the front face of the piston 29 are always at atmospheric pressure since the space within the hollow boss 21 is in communication with the atmosphere through a port 39. The space 70 into which the piston rod 31 extends is closed, but not hermetically, by a dust cover 71. The space 70 is therefore at about atmospheric pressure.

In the position shown in Fig. 1 the valves are blocked and the clearance space at the rear face of the smaller piston 29 is in communication with the reservoir through a supply port 40 in the member 31, and through the central aperture and the aperture 37 in the bolt 30. Assuming that the reservoir is initially at atmospheric pressure, the air pressures on the piston assembly 23, 29, 31 are balanced and the whole force exerted by the spring 38 urges the blocking plates 24 and 25 against the valve faces.

When the compressor is started, the pressure in the reservoir rises, since some air is delivered through the valves 24 and 25 even when they are blocked. The reservoir pressure acts upon the larger piston 23 in a sense opposing the spring 38 and acts upon the smaller piston 29 in a sense assisting the spring 38. In eiect, therefore, (neglecting the effect of atmospheric pressure on the small area of the piston rod 31) the spring 38 is opposed by the reservoir pressure acting upon a piston area equal to the dierence between the areas of the larger and smaller pistons. As the reservoir pressure rises, therefore, the blocking force exerted by the spring 38 and the air pressures is progressively reduced until a condition is reached when the piston assembly begins to move t the right in Fig. 1. A very small movement of the member 31 results in the supply port 40 moving out of communication with the clearance space at the rear face of the smaller piston 29 and in this clearance space being placed in communication with the atmosphere through a slot 41 in the member 31 and a port 42 in the boss 33. There is thus a sudden drop in the force tending to urge the piston assembly to the left in Fig. 1 with the result that the piston assembly pops, or moves suddenly, into its extreme right position. l The springs beneath the blocking plates 24 and 2S are then free to move these plates fully away from their blocking positlon into their un-blocking position, this movement being assisted by the flow of air through the valves.

A spring-loaded slipper valve 43 is provided to seal the supply port 40 when the piston assembly is in the fully lunblocked condition, that is in the normal runmng condition, in order to avoid unnecessary leakage from the reservoir.

When the pressure in the reservoir falls, at a certain value of this pressure the piston assembly moves to the left in Fig. l until the clearance space at the rear of the piston 29 is cut ott from the atmosphere and put iutQ communication with the reservoir through the supply port 40. When this happens what is called the popopen point is reached and the assembly moves rapidly into the fully-blocking position.

It may be noted that the pop-open point is determined by the pressure acting upon an area equal to the diiference between the areas of the larger and smaller pistons whereas the pop-shut point is determined by the pressure acting on the whole area of the larger piston. For this reason the pop-open point is at a higher pressure than the pop-shut point.

In one example the larger piston 23 has a diameter of 6.7 and the smaller 29 a diameter 3.7". The outlet area of each of the two delivery valves is 1.8 x 3.1 and the blocking plates 24 and 25 overlap this by about M3 or more all round. The spring has a rate of about 1200 lb./inch, and the spring load in the blocked position is about 2100 lb. Thus its size and weight is much less than for the corresponding springs in known blocking means for streamline valves.

In order to increase the effective area of the smaller piston 29, the space 70 may be hermetically sealed by the cover 71 and a hole may be provided between the space 70 and the clearance space behind the piston 29. The same pressures then act upon the rear face of the smaller piston and the end of the piston rod 31. In this way the pressure of the pop-open can be somewhat increased.

In the arrangement described with reference to Figs. l and 2 the pressure behind the larger piston 23, that is the pressure within the hollow boss 21, is maintained at atmospheric and the pressure behind the smaller piston 29 is varied between reservoir pressure and atmospheric pressure. The reverse arrangement may, however, be used. Some means are then provided to vent the clearance space behind the smaller piston 29 (and it' desired also the space 70) to atmosphere and suitable ports operated by the movements of the piston assembly serve to connect the space within the boss 21 either to reservoir pressure or to atmosphere. The port 39 is of course not provided.

Fig. 3 shows schematically a servo-valve control for a blocking piston. The separation of the control means from the compressor in this embodiment avoids any risk of the control means becoming fouled or overheated by their proximity to the engine.

In Fig. 3, a compressor piston 45 is shown in the compressor cylinder 10 which communicates through delivery valves 14 and 15 with the interior of a casing 19 communicating with a reservoir 46. A blocking piston 47 operates in a cylinder formed within the casing 19. In order to block the valves 14 and 15 springs 48 urge the piston 47 to the left and cause projections 49 to engage suitable blocking plates (not shown) over the valves 14 and 15.

The control means comprise a cylinder 50 in which operates a piston 51 carrying a slide valve 52. The piston 51 is urged toward the left by a spring 53 and is shown in its left-most position corresponding to full blocking of the valves 14 and 15. The clearance space 54 over the piston 51 is connected by a pipe 55 with the reservoir 46 and the space behind the piston 47 is connected by a pipe 56 with ducts 57 and 58 in a member 59 co-operating with the slide valve 52. A port 60 in the slide valve 52 communicates through an aperture in the piston 51, space 54, and pipe 55 with the reservoir 46. A port 61 in the slide valve 52 is in communication with the atmosphere. A smaller piston 62 connected to the larger piston 51 operates in a cylinder 63 and the clearance space 64 behind the smaller piston communicates through a duct 65 with the duct 57. 'Ihe clearance space 66 behind the larger piston 51 is connected to the atmosphere through a port 67.

When the compressor is started the reservoir pressure rises and this pressure in the space 54 acts against the spring 53. The reservoir pressure also acts through the port 60 and the duct 65 upon the rear face of the smaller piston 62. Thus, as in the embodiment of Figs. l and 2, the reservoir pressure acts in effect upon an area equal to the difference between the areas of the larger and smaller pistons 51 and 62 in opposition to the spring 53.

In addition, the reservoir pressure acts through the duct 57 and the pipe 56 upon the rear face of the piston 47 so that in the condition being considered equal pressures act upon the two sides of the piston 47 and the whole force exerted by the spring 48 is applied to block the valves 14 and 15.

At a predetermined value of the reservoir pressure, the slide valve 52 moves to the right sufficiently to disconnect the duct 57 from the port 60 and shortly afterwards connects the duct 58 with the port 61 and therefore to the atmosphere. The piston assembly 51, 62 with the slide valve 52 then pop into their iin-blocking position and the whole reservoir pressure is suddenly brought to act upon the front face of the piston 47 against the spring 43, thus unblocking the valves 14 and 15.

I claim:

l. A free-piston air compressor comprising a compressor cylinder, a compressor piston operating in said cylinder, an air reservoir, a passage connecting said cylinder to said reservoir, at least one delivery valve in said connecting passage, means for blocking said delivery valve, means for exerting force upon said blocking means to urge said blocking means into their blocked position, and blocking control means for controlling the application of said force, said blocking control means comprising two control cylinders of different diameters, a control piston operating in each control cylinder, said control pistons being coupled together and being movable in said control cylinders between a blocked and an un-blocked position, spring means urging said control pistons toward their blocked position, means connecting the front face of the larger control piston to said reservoir, pressure on this face opposing said spring means, means connecting the rear face of one of said control pistons to the atmosphere, and means operated by displacement of said control pistons to connect the rear face of the other of said control pistons to said reservoir in the blocked position of said control pistons and to connect the rearV face of said other piston to the atmosphere in the un-blocked position of said control pistons, pressure on the rear face of said other piston assisting said spring means.

2. A free-piston air compressor comprising a compressor cylinder, a compressor piston operating in said cylinder, an air reservoir, a passage connecting said cylinder to said reservoir, at least one delivery valve in said connecting passage, means for blocking said delivery valve, means for exerting force upon said blocking means to urge said blocking means into their blocked position, and blocking control means for controlling the application of said force, said blocking control means comprising two control cylinders of diierent diameters, a control piston operating in each control cylinder, said control pistons being coupled together and being movable in said control cylinders between a blocked -and an un-blocked position, spring means urging said control pistons toward their blocked position, means connecting the front face of the larger control piston to said reservoir, pressure on this face opposing said spring means, means connecting the rear face of the larger control piston to the atmosphere, and means operated by displacement of said control pistons to connect the rear face of the smaller control piston to said reservoir in the blocked position of said control pistons and to connect the rear face of said smaller piston to the atmosphere in the un-blocked position of said control pistons, pressure on the rear face of said smaller piston assisting said spring means.

3. A freeapiston air-compressor according to claim 2, wherein the last-named means comprise ports operated by displacement of said control pistons.

4. A free-piston air-compressor according to claim 2, wherein said spring means exert force upon said blocking means.

No references cited. 

